In this renewal proposal, we continue our work demonstrating the critical functional role of pericyte and endothelial cell (EC)-derived proteinase inhibitors to control the process of endothelial tube stabilization in 3D extracellular matrix environments. Our overall hypothesis is that the proteinase inhibitors, TIMP-3 and TIMP-2, possess stabilizing properties due to their ability to suppress both promorphogenic and proregression stimuli which are necessary for tube stabilization. During this last funding period, we have made essential progress in demonstrating the ability of pericytes to stabilize EC-lined tubes and the role of TIMPs during this process, have developed novel EC-pericyte tube coculture models to identify molecular requirements for these events, and have identified critical EC targets of TIMP-3 and TIMP-2 such as MT1-MMP, MMP-1, MMP-10 and ADAM- 15 which mediate their influence. In addition, we have made significant advances in our ability to; analyze gene expression and function in ECs versus pericytes to identify the molecular requirements for this process; analyze the influence of extracellular matrix remodeling changes such as identifying a requirement for EC- pericyte interactions to catalyze vascular basement membrane matrix assembly; identify mechanisms whereby pericytes are recruited to EC-lined tubes through PDGF-BB and HB-EGF mediated signaling; and determine how critical growth factors such as TGF-beta affect this process. We propose three specific aims which are; Specific Aim #1: To determine the molecular mechanisms by which pericytes are able to catalyze endothelial basement membrane matrix assembly to facilitate vascular stabilization. Specific Aim #2: To define the mechanisms controlling how pericytes are specifically recruited to endothelial cell-lined tubes and how they dynamically scan the endothelial basal surface through migratory events to regulate tube stabilization. Specific Aim #3: To define how pericyte recruitment to developing vascular tubes leads to stabilization through attenuation of regression stimuli mediated by the ubiquitous cytokine, TGF-beta.